Space propulsion, surface cleaning, ion implantation, and high energy accelerators use ion beam sources. These beam sources typically use two or three closely spaced multiple-aperture electrodes to extract ions from a source and eject them in a collimated beam. These electrodes are called "grids" because they have a large number of small holes. Typically, tile grids are made from molybdenum. A series of grids constitute an electrostatic ion accelerator and focusing system commonly referred to as the "ion optics."
Ion beam sources designed for spacecraft propulsion, that is, ion thrusters, should have long lifetimes (10,000 hours or more), be efficient, and be lightweight. These factors can be important in other applications as well, but they are not as critical to successful use as they are for ion thrusters. Ion thrusters have been successfully tested in space, and show promise for significant savings in propellant because of their high specific impulse (an order of magnitude higher than that of chemical rocket engines). They have yet to achieve any significant space use, however, due in part to lifetime limitations imposed by grid erosion and to performance constraints imposed by thermal-mechanical design considerations resulting from the use of metallic grids.
A typical configuration of an ion thruster is known as an electron bombardment ion thruster. In an electron bombardment ion thruster, electrons produced by a cathode strike neutral gas atoms introduced through a propellant feed line. The electrons ionize the gas propellant and produce a diffuse plasma. In other types of ion thrusters, known as "radio frequency ion thrusters," the propellant is ionized electromagnetically by an external coil, and there is no cathode. In both cases, an anode associated with the plasma raises its positive potential. To maintain the positive potential of the anode, a power supply pumps some of the electrons that the anode collects from the plasma down to ground potential. These electrons are ejected into space by a neutralizer to neutralize the ion beam. Magnets act to inhibit electrons and ions from leaving the plasma Ions drift toward the ion optics, and enter the holes in a screen grid. A voltage difference between the screen grid and an accelerator grid accelerates the ions, thereby creating thrust. The screen grid is at the plasma potential, and the accelerator grid is held at a negative potential to prevent downstream electrons from entering the thruster. Optionally, the optics can include a decelerator grid located slightly downstream of the accelerator grid and held at ground potential or at a lesser negative potential than the accelerator grid to improve beam focusing and reduce ion impingement on the negative accelerator grid.
A primary life limiting mechanism in ion thrusters is erosion of the ion optics (i.e., the grids) from ions impacting the grid material and sputtering it away. In ion thrusters, slow moving ions are produced within and downstream of the ion optics by a charge exchange (i.e., electron hopping) from neutral propellant atoms to fast moving ions that pass close by. These "charge exchange" ions are attracted to the accelerator grid and strike it at high energy, gradually eroding it away. The screen grid also experiences some erosion, mostly on the upstream side. This erosion of both the screen grid and accelerator grid eventually produces additional holes in the grids, causing them to cease functioning properly. Grid erosion is the primary life-limiting mechanism for ion optics.
A principal factor affecting both the efficiency and the weight of ion thrusters is how closely and precisely the grids can be positioned while maintaining relative uniformity in the grid-to-grid spacing under conditions conducive to significant thermal distortion. In the past, this factor has limited the maximum practical diameter of ion thrusters, which severely constrains taking advantage of scale effects that theoretically would improve efficiency, thrust-to-weight ratio, and reliability.
Molybdenum ion thruster grids are precisely hydroformed into matching convex shapes. The apertures are chemically etched. The convex shapes provide a predictable direction for the deformation that occurs due to thermal expansion when a thruster heats in operation. Changes in the actual spacing and the uniformity of spacing over the grid surfaces between the molybdenum grids is unpredictable and uncontrollable. The thermal expansion distribution is complex.
The changes in spacing that occur adversely effect performance. Although techniques have been developed to compensate for such changes, the unpredictable and nonuniform nature of the changes prevents complete compensation.
In ion beam sources used for terrestrial applications, today's grids are sometimes made of graphite, which expands much less than molybdenum when heated. Graphite is, however, relatively flexible and fragile and is not suitable for beam sources larger than about 15-20 cm in diameter, or for ion thruster grids, which are subject to severe vibration during launch from Earth.
It is desirable to have a screen grid and accelerator grid that have lifetimes of 10,000 to 20,000 hours for use in a variety of space propulsion applications. Such grids should also have an increased efficiency and should be lightweight for space applications. Additionally, the screen grids should allow for the construction of an ion optics set wherein the magnitude and uniformity of the spacing between the grids can be precisely predicted and maintained over the temperature range and pattern of differential surface heating the grids experience in use.